The present invention relates to a semi-automatic transmission control and particularly to such control for preventing the shifting of transmission gears when such action would place the engine in an unfavorable operating condition with respect to the engine speed and load condition.
In addition to the steering and brake systems being power operated, the trend in truck type automotive vehicles appears to be toward power assist shifting for transmissions, thus reducing the effort required to change gears. Power shift arrangements make possible the linear alignment of the gears, as opposed to the conventional H-formation. This facilitates gear shifting where large numbers of gears are available, such as in trucks, but introduces a danger, however, in that one or even several gears could be accidentally skipped. While the skipping of gears is sometimes desirable, engine damage due to overspeeding can result if a relatively fast moving vehicle is down-shifted to too low a gear. For example, truck engines can be oversped and destroyed at 3500 rpm. The maximum permissible speed; therefore, would be on the order of 3100 rpm, and the nominal speed approximately 2400 rpm. The optimum-engine operating range, therefore, is approximately 70% to 100% of the rated speed.
Accidental shifting into too high of a gear is also undesirable, since such a condition unduly overloads the engine, particularly when pulling a grade.